Electrochemical gas sensors using microelectrodes have a number of benefits, such as the ability to use fast scanning voltammetric measurements, utilizing features such as convergent diffusion and potentially being simpler to construct than conventional gas diffusion electrodes. However they also suffer from disadvantages. In particular it is more difficult to achieve a reliable 3-phase region, i.e., a region where gas does not have to diffuse a long distance through the electrolyte before reaching the sensing electrode(s).
The analyte gas can dissolve in the electrolyte and give rise to a background current which remains after the target gas is removed, resulting in slow response times and background current errors.
This issue is normally overcome in conventional gas sensors by ensuring that all of the target gas is consumed by the sensing electrode (e.g., via capillary limitation of flux to a gas diffusion electrode). However the design of gas diffusion electrodes is complex, especially when using non-aqueous electrolytes such as ionic liquids, and the resulting large surface area and hence double layer capacitance means that dynamic electrochemical techniques such as scanning voltammetry are not feasible.
An example of the current state of the art in microband electrodes are those manufactured by Nanoflex® (http://www.nanoflex.com/Products/Product/Platinum_Substrate) which comprise band electrodes within wells of micrometer dimensions. Previously people have also used line electrode devices deposited on ionic conducting substrates (Kirsi Wallgren, PhD thesis 2005, University of Nottingham and references therein). Microband electrodes can be made via sandwich structures using, for example, micromachined silicon layers or line electrodes on the surface of a substrate. These approaches do not address the issue of ensuring easy gas access to the sensing electrode, since it is immersed within the electrolyte (which may be either a solid or liquid).
In accordance herewith, methods and structures are provided for utilizing microelectrodes and particularly microband electrodes in such a way as to ensure rapid gas access to the sensing electrode(s) and/or to compensate or correct the sensor behavior for variations in the degree to which the sensing electrode is wetted. The attached figures illustrate various aspects of embodiments hereof.